Determining a location of a first object relative to a second object enables a position of the first object (e.g., a medical instrument) that is partially in the second object (e.g., a body structure such as a bone) to also be determined. In the field of medicine, in particular in operations, situations can arise wherein information on the location of a part of an instrument relative to a body structure cannot be directly detected, or only detected with significant difficulty. If, for example, an instrument is introduced into a body structure and the restricted spatial conditions prohibit a marker or reference star from being attached to the instrument, then determining the position of hidden parts of the instrument relative to the body structure becomes problematic.